Influencing factors of pressure pain hypersensitivity of the upper trapezius in food service workers with nonspecific neck/shoulder myofascial pain: A cross-sectional study

It is unclear which factors contribute to the developing pressure pain hypersensitivity of the upper trapezius, a type of neurophysiological hyperexcitability. The present study investigated the relationship between physical and psychological factors and pressure pain hypersensitivity of the upper trapezius for each sex. In total, 154 individuals with neck/shoulder myofascial pain participated, among 372 food service workers. Participants completed a questionnaire (Beck Depression Inventory, and Borg Rating of Perceived Exertion scale) and were photographed to measure posture. Pressure pain sensitivity, 2 range of motions (cervical lateral bending and rotation), and 4 muscle strengths (serratus anterior, lower trapezius [LT], biceps, and glenohumeral external rotator) were measured by a pressure algometer, iPhone application, and handheld dynamometer, respectively. For each sex, forward multivariate logistic regression was used to test our a priori hypothesis among selected variables that a combination of psychosocial and physical factors contributed to the risk for pressure pain hypersensitivity. In multivariate analyses, LT strength (odds ratio = 0.94, 95% confidence interval = 0.91–0.97, P = .001) was the only significant influencing factor for pressure pain hypersensitivity in men. Dominant painful ipsilateral cervical rotation range of motion (odds ratio = 0.96, 95% confidence interval = 0.92–0.99, P = .037) was the only influencing factor for pressure pain hypersensitivity in women. LT strength and dominant painful ipsilateral cervical rotation range of motion could serve as guidelines for preventing and managing pressure pain hypersensitivity of the upper trapezius in food service workers with nonspecific neck/shoulder myofascial pain.

Medicine myofascial trigger points, which are hyperirritable regions of tenderness in the taut bands of skeletal muscles [8] that become painful when stimulated (e.g., via compression or other mechanical stimulations) and can contribute to the generation of pain, motor dysfunction, and autonomic responses. [8][9][10] Among postural muscles, the upper trapezius (UT) muscle is most affected by MP. [7,11,12] Tenderness over muscles is a common clinical finding in painful conditions of presumed muscular origin. [13,14] Pressure pain sensitivity (PPS) is a quantitative sensory test for assessing pain sensitivity in deep tissues. [15] This neurophysiological test may be suitable to measure PPS and tissue tenderness because these conditions are believed to reduce the test values. [15] Although the pathophysiologic mechanism of MP has not been identified, it may involve central sensitization (hyperresponsiveness and hyperexcitability of the central nervous system). [16,17] However, it is unclear which factors increase the risk of developing pressure pain hypersensitivity (PPH) in terms of neurophysiological hyperresponsiveness and hyperexcitability. Suggested factors include individual factors (e.g., sex), [18] physical factors (e.g., posture), [19] and psychosocial factors (e.g., depression and stress). [20] Previous studies have investigated the influences of individual factors on PPS. Thus, there is a need to investigate the combined influences of multiple factors on the PPS.
Physical influencing factors are useful and potentially could be modified with interventions such as exercise. [21] Conversely, individual characteristics (e.g., sex and age) cannot be modified.
To determine a specific management approach for neck/shoulder pain, Donatelli [22] proposed examination of the following aspects: cervical and shoulder posture, muscle length, rotator cuff muscle strength, and scapular rotator strength. Psychosocial factors and increased pain sensitivity are phenotypic domains related to the risk of developing chronic musculoskeletal pain. [23] Increased PPS may be indicative of altered central pain mechanisms by psychosocial factors. [24] Psychosocial influencing factors include pain catastrophizing, kinesiophobia, anxiety, depression, and stress associated with boring and monotonous tasks, low social support, and low job satisfaction. [25,26] Psychological, biological, and social domains could explain differences in pain severity and perception between men and women with MP. [18,27] Concerning sex differences in pain outcomes, PPS has demonstrated the greatest effect size [28] and women are more sensitive to pressure pain than men. [29] Because of sex differences in pain perception and PPS, as well as differences in food service tasks, the relative contributions of physical and psychological influencing factors to PPH should be identified for each sex.
Therefore, the present study investigated differences in physical and psychosocial factors between participants with and without PPH for each sex and the relationship between physical and psychological factors and PPH for each sex.

Participants
Participants were recruited through a questionnaire to confirm their experience of neck/shoulder MP as FWs. In total, 154 individuals with neck/shoulder MP participated, from among 372 FWs in a theme park. A flowchart for recruitment of the participants is shown in Figure 1. Inclusion criteria were nontraumatic neck/shoulder pain, >6 months of work in food service, presence of neck/shoulder pain for ≥3 months, and visual analog scale score >30 mm. Exclusion criteria were shoulder fractures, a prior diagnosis of shoulder instability, a history of surgery in the shoulder, any systemic disease, untreated psychiatric condition, examination suggesting the presence of neurological diseases or internal diseases, hypertension (resting systolic blood pressure >150 mm Hg or diastolic blood pressure >90 mm Hg), and/or pregnancy. Participant characteristics are shown in Table 1. The experimental protocol was established according to the ethical guidelines of the Helsinki Declaration. The study protocol was   [30,31] PPS was defined as the lowest pressure at which the sensation of pressure turned to slight pain or discomfort. [18,30,31] The mean value of 3 trials was calculated and used for the main analyses. A 1 minute resting period was allowed between each recording. Both the participant and examiner were blind to force readings during the assessment. A standard metronome was also used to control the rate of increase in pressure. Men with PPS <2.9 kg/cm 2 in the UT and women with PPS <2.0 kg/cm 2 in the UT were presumed to have PPH. [32] 2.2.2. Psychological domain: depression. The Beck Depression Inventory (BDI) is widely used to measure depression. Previous studies have supported the suitability of the BDI in assessing depression in patients with chronic musculoskeletal pain. [33][34][35] The BDI consists of 21 items based on symptoms and attitudes. [36] Statements were ranked to indicate the range of depression severity from neutral to maximal.

Physical domains.
Exertion of work intensity was measured using the Borg rating of perceived exertion scale.
Participants were asked to self-rate their exertion of work intensity on a scale between 6 and 20. [37] For cervical range of motion (ROM), the dominant painful contralateral cervical side-bending and dominant painful ipsilateral cervical rotation ROM were measured using an iPhone with Clinometer and Compass applications [38,39] (Fig. 2). Using a belt strap, participants were blocked from performing trunk and shoulder movements during measurements of cervical lateral bending and rotation movement. The measurements of cervical ROM were made for the total range (i.e., difference between initial and final measures). The mean value of 3 trials was calculated and used for the main analyses.
For muscle strength, serratus anterior, lower trapezius (LT), biceps, and glenohumeral external rotator strengths were measured using a handheld dynamometer (JTECH Medical, Salt Lake City, UT) in the dominant painful side [39] (Fig. 3). The unit of measurement was a Newton (N) generated by isometric contraction. Muscle strength values were normalized according to participant body weight. The mean value of 3 trials was calculated and used for analyses. Serratus anterior strength was measured in scapular protraction and the shoulder was flexed to 125°. [40] Participants were instructed to hold the upper extremity position while the examiner provided a downward force with the handheld dynamometer immediately over the distal humerus. In the prone position, LT strength was measured with the upper extremity diagonally overhead, in line with the LT fibers. [41] The handheld dynamometer was applied to the distal one-third of the participant's radial forearm, and force toward the floor was applied by the examiner. Biceps strength was measured with participants in the sitting position with their elbow flexed to 90°. [41] The handheld dynamometer force sensor was applied to the distal one-third of the participant's forearm, and force toward the floor was applied by the examiner. Glenohumeral external rotator was measured in the side-lying position with the shoulder flexed and internally rotated to 90°, and the elbow flexed to 90°. [39] Then the dynamometer was applied to the distal one-third of the participant's radial forearm, and force toward the floor was applied by the examiner.
For posture analyses, forward head posture, rounded shoulder angle, shoulder slope angle, and scapular downward rotation ratio were measured using kinematic analyses of photographs using ImageJ software (National Institutes of Health, Bethesda, MD). Forward head posture was quantified by the craniovertebral angle (angle between the horizontal line passing through C7 and a line extending from the tragus of the ear to C7). The rounded shoulder angle was quantified using the angle (θ) between 2 lines (from a horizontal line in the medial roots of the scapula to the acromion, and from the root of the scapula to the acromion). The angle θ, composed of the 2 distances, is 1 apex of a right-angled triangle. Therefore, 90θ was defined as the rounded shoulder angle. Shoulder slope angle was quantified by the angle between 2 lines (a horizontal line with the acromion and a line between the spinous process of the seventh cervical vertebrae and acromion). Scapular downward rotation ratio was quantified by the ratio between 2 lines (a vertical line from the center to the root of the scapula, and a vertical line from the center to the scapular inferior angle).

Procedures
The present study was performed from March 2016 to November 2016. Participants were assessed at the work conditioning center in a theme park. Variables were measured in the following order: psychological and physical domains (posture, ROM, and strength). Participants were asked to complete a questionnaire (age, sex, BDI, and Borg rating of perceived exertion scale) and then were photographed to measure posture. PPS, 2 ROMs (cervical lateral bending and rotation), and 4 muscle strengths (serratus anterior, LT, biceps, and glenohumeral external rotator) were measured in that order. Two testers performed the measurements separately for interrater reliability. One tester screened for and assessed PPS, and the other tester measured the independents variables. The participants were blinded to the PPS data.

Statistical analyses
The Kolmogorov-Smirnov Z test was used to assess the assumption of distribution normality. Demographic characteristics are shown as means. Independent t tests were used to compare psychological and physical domains between participants with PPH and participants without PPH, and to identify significant variables for cutoff in each sex because of sex differences in demographic and clinical features. Variables with significant differences between participants according to PPH status in men and women were selected. For each sex, variables associated with PPH were selected from univariate analyses. Finally, for each sex, forward multivariate logistic regression was used to test our a priori hypothesis among selected variables that a combination of psychosocial and physical factors contributed to the PPH. The analyses were adjusted for previously established covariates of age and body mass index. Goodness-of-fit was calculated using the Hosmer-Lemeshow test. Statistical analyses were conducted using SPSS Statistics (version 18.0; IBM Corp., Armonk, NY) and the significance level was set at P < .05. Table 2 shows comparisons of psychological and physical domains between men with PPH and men without PPH. There were no significant differences in BDI, Borg rating of perceived exertion scale, forward head posture, rounded shoulder angle, shoulder slope angle, or scapular downward rotation ratio   Table 3 shows comparisons of psychological and physical domains between women with PPH and women without PPH. Most variables were not significantly different, but dominant painful ipsilateral cervical rotation ROM (P = .033) was significantly greater in women without PPH than in women with PPH.

Multivariate prediction model for PPH of UT in men and women
By comparison of psychological and physical domains between participants with PPH and participants without PPH, the following variables were selected: dominant painful contralateral cervical side-bending, dominant painful ipsilateral cervical rotation ROM, serratus anterior, LT, biceps, and glenohumeral external rotator strength. The results of univariate analyses of predictors of PPH of UT in men and women are shown in Table 4  The results of adjusted multivariate analyses of predictors of PPH of UT in men and women are shown in Table 5 and Supplemental Digital Content 1, Supplemental Digital Content, http://links.lww.com/MD/G968. In the adjusted multivariate model, LT strength (OR = 0.94, 95% CI = 0.91-0.97, P = .001) was the only significant influencing factor for PPH of the UT in men. In addition, dominant painful ipsilateral cervical rotation ROM (OR = 0.96, 95% CI = 0.92-0.99, P = .037) was the only influencing factor for PPH of the UT in women. Goodness-of-fit statistics indicated that model fitting was appropriate for each sex-adjusted regression model (men: P = .290, women: P = .061).

Discussion
PPS can result from impairments at multiple levels throughout the neuromuscular system. [42] There is increasing evidence that changes in pain processing may enhance sensitivity to noxious stimuli among individuals with chronic pain, compared to painfree controls. [14,43] The present study investigated whether physical and psychological domains were related to PPH of the UT in each sex among FWs with nonspecific neck/shoulder MP, because biological differences have been suggested to cause sex differences in pain perception. [44][45][46] LT strength and dominant painful ipsilateral cervical rotation ROM were characterized as influencing factors of PPH of the UT in male and female FWs with nonspecific neck/shoulder MP in adjusted multivariate analyses. Although our interpretations of causality are limited because of the cross-sectional study design, the LT strength and dominant painful ipsilateral cervical rotation ROM identified in the present study could be useful for establishing guidelines for the prevention and management of PPH in FWs with nonspecific neck/shoulder MP. Concerning LT strength as influencing factor for PPH, scapulothoracic muscle imbalances could be the cause of impaired biomechanics, postural adaptations, and neck/shoulder pain. [47,48] These imbalances may occur when the UT becomes tight and the LT becomes weak. [49,50] Conversely, LT weakness could result in UT overload because of poor scapular mechanics (e.g., increasing scapular elevation and decreasing scapular upward rotation and posterior tilting) [47,48] and weakly synergistic acceleration of UT overactivation (e.g., involving the serratus anterior and LT). [51] LT strength was significantly different between the ipsilateral (mean ± standard deviation (SD): 21.8 ± 10.0 N) and contralateral sides (mean ± SD: 25.7 ± 11.5 N) in individuals with unilateral neck and shoulder pain. [50] In the current study, LT strength in male FWs with PPH was 31.61% ± 15.58% normalized by body weight. Before LT strength was divided by body weight, LT strength was 23.50 N, which was similar to the results of a previous study involving individuals with neck/ shoulder pain. [50] However, Shahidi et al [42] investigated physical influencing factors of chronic neck pain. They found that LT strength was not an influencing factor, using a multivariate Table 4 Predictors of pressure pain hypersensitivity using selected variables: results from univariate analysis.

Sex
Variables   prediction model that involved cervical active ROM, cervical muscle strength and endurance, and scapular muscle strength.
Although this explanation is limited by the cross-sectional study design, LT could be linked to PPH of the UT and could potentially be weaker in terms of PPH of the UT. The process may function in an inverse manner. Cervical mobility as a influencing factor for neck/shoulder pain has been suggested in prospective studies of other populations, but the results have been conflicting. Reduced cervical flexion mobility was more likely to cause neck/shoulder pain in laundry workers (risk ratio: 3.1; 95% CI = 1.2-8.3) [52] and increased cervical flexion-extension mobility was protective against neck/shoulder pain in office workers (hazard ratio: 0.97; 95% CI: 0.94-0.99). [53] With respect to dominant painful ipsilateral cervical rotation ROM as a influencing factor for PPH, cervical rotation ROM is related to pain intensity in patients with chronic neck/shoulder pain. [54,55] Moreover, patients with nonspecific neck/shoulder pain show less cervical rotation ROM, compared to asymptomatic controls. [56,57] Reduced extensibility of upper quadrant neural structures evaluated by the median nerve tension test has been related to decreasing UT length. [58] Furthermore, the presence of PPH in the UT was associated with cervical intervertebral joint dysfunctions. [59] Although interpretations are restricted by the cross-sectional study design, dominant painful side ipsilateral cervical rotation ROM could be linked to PPH of the UT and could potentially cause shortness involving PPH of the UT, or the process could function in an inverse manner. UT length affects ipsilateral cervical rotation ROM and contralateral cervical side-bending ROM because of the muscle attachment locations. [60] Thus, tenderness or PPH of the UT could affect the restriction of cervical ipsilateral rotation ROM. Conversely, reduced UT length could affect PPH by scapular dyskinesis (e.g., scapular elevation during arm lifting). [60] UT shortness and scapular dyskinesis could generate reduced activity of the serratus anterior and/or LT, as well as enhanced activity of the UT, resulting in UT overactivation. [61,62] The psychological domain (depressed mood), as measured using the BDI, was not significantly different between FWs with PPH of the UT and FWs without PPH of the UT in both men (P = .988) and women (P = .666). This might have been due to limited statistical power resulting from the small sample size in this study. Psychological depressed mood was reportedly associated with an enhanced risk for neck pain in office workers (OR = 3.36; 95% CI = 1.10-10.31; P = .03) [42] and others. [63,64] Although it is difficult to directly compare our findings with the results of previous studies, a possible reason for exclusion of the psychological domain from the variable selection process was that the psychological domain could more weakly influence PPH among workers with repetitive and high physical load tasks, compared to white-collar office workers. Furthermore, physical domains of cervical and scapular posture were not significantly different between FWs with PPH of the UT and FWs without PPH of the UT in both men and women. Forward head posture [65,66] and scapular posture [19] have been previously associated with neck/ shoulder pain. Because cervical and scapular posture are static characteristics, dynamic physical domains could more strongly influence PPH among workers with repetitive and high physical load tasks, rather than static physical domains.
The main limitation of the present study was its small sample size measured in 1 theme park. Furthermore, this study involved a relatively homogeneous sample of FWs. Future studies are necessary to determine whether the influencing factors identified in this study can be generalized to other demographic populations and professions. In addition, the design of present study was cross-sectional study. It is difficult to determine the direction of association between the variables in a cross-sectional study design. Thus, longitudinal study is necessary to determine causality of PPH in a future study. Furthermore, we measured depression in only the psychological domain. It is necessary to confirm the relationship among various psychological domains (anxiety, embarrassment, and job stress), social domains (race, culture, years of work experience) and musculoskeletal pain in future studies. Finally, future studies should also determine whether improvements in LT strength and cervical rotation ROM are effective for reducing PPH among individuals with neck/shoulder MP.

Conclusion
The present study investigated physical and psychological influencing factors of PPH of the UT in each sex among FWs with nonspecific neck/shoulder MP. LT strength and dominant painful ipsilateral cervical rotation ROM were influencing factors of PPH of the UT in men and women in adjusted multivariate analyses. Improvements in LT strength and dominant painful ipsilateral cervical rotation ROM may be protective against PPH in FWs with nonspecific neck/shoulder MP.